Cutting insert, cutting tool, and method of manufacturing a machined product

ABSTRACT

A cutting insert has a cutting edge having a first corner cutting edge, a flat cutting edge, a connecting edge, a major cutting edge, and a second corner cutting edge in the order named. The major cutting edge has an upwardly protruding curvilinear portion, a first straight line portion extending from the curvilinear portion toward the connecting edge, and a second straight line portion extending from the curvilinear portion toward the second corner cutting edge. The flat cutting edge and the major cutting edge have a straight line shape, and the connecting edge has an outwardly protruding curvilinear shape in a top view. The flat cutting edge, the connecting edge, and the first straight line portion are located on a straight line in a side view.

TECHNICAL FIELD

The present invention relates to a cutting insert, a cutting tool, and amethod of manufacturing a machined product.

BACKGROUND ART

Cutting inserts disclosed in, for example, International Publication No.2013/029072 (Patent Document 1) and International Publication No.2004/080633 (Patent Document 2) have conventionally been known as acutting insert for use in a cutting process of a workpiece. The cuttinginserts disclosed in Patent Documents 1 and 2 are used in a millingprocess, such as a face milling process or an end milling process.

The cutting inserts respectively disclosed in Patent Documents 1 and 2are provided with a flat cutting edge being parallel to a surface of theworkpiece in order that a machining surface, namely, a surface of theworkpiece to be machined by a cutting process is made into a smoothfinished surface. Although the cutting insert disclosed in PatentDocument 1 has the flat cutting edge, a cutting edge angle of a majorcutting edge is large. The cutting edge angle is an inclination angle ofthe cutting edge of the cutting insert being attached to a holder withrespect to the machining surface of the workpiece. That is, with thecutting insert disclosed in Patent Document 1, chips to be generatedduring cutting of the workpiece have a large thickness, and the majorcutting edge is subjected to a large cutting resistance. Consequently,heat is apt to stay in the major cutting edge, and then the majorcutting edge may fracture. It is therefore necessary to decrease a feedper revolution that is an amount of movement of an edge of a cuttingtool per revolution. It is possible to decrease the cutting edge angleand increase the feed per revolution by making the flat cutting edge andthe major cutting edge into a continuous straight line shape as in thecutting insert disclosed in Patent Document 2.

FIG. 1 shows a situation where the milling process is carried out usingthe cutting insert disclosed in Patent Documents 1 and 2. As shown inFIG. 1, the workpiece W is to be machined by rotating a plurality of thecutting inserts 1′ along an outer periphery of the holder 101′ havingthe plurality of the cutting inserts 1′ attached thereto, while movingthe holder 101′ in one direction. During the machining, a rotationcenter axis Y1 of the holder 101′ is inclined at an angle θ toward anadvance direction L of the holder 101′ with respect to a vertical axisY2 relative to the machining surface of the workpiece W. In order toobtain a satisfactorily machined surface, a front edge A cuts theworkpiece W with the holder 101′ inclined at the angle θ during themachining of the workpiece W, and a rear edge B needs to be moved awayfrom a finished surface. However, the holder 101′ is subjected to athrust force P applied vertically from the machining surface to thefront edge A. The thrust force P exerted on the front edge A can causethe front edge A to move up, and the rear edge B can move down byreaction thereof. Hence, when the thrust force P is large, there is arisk that the rear edge B significantly moves down and contacts with thefinished surface.

After the machining surface is machined into a smooth finished surfacewith less unevenness by using the flat cutting edge, the occurrence ofdamage to the finished surface by the rear edge B becomes a cause ofpoor machined products. Particularly, in the cutting insert disclosed inPatent Document 2, there remains a risk that the finished surface isdamaged because the cutting edge angle of the major cutting edge beingcontinuous with the flat cutting edge is small and the thrust force islarge.

The present invention has been made in view of the above problems, andit is an object of the present invention to provide a cutting insert, acutting tool, and a method of manufacturing a machined product, all ofwhich make it possible to suppress damage to the finished surface whilereducing the risk of fractures in the cutting edge.

SUMMARY OF THE INVENTION

A cutting insert according to an embodiment of the present inventionhas: an upper surface with a polygonal shape which has a side part, anda first corner part and a second corner part each being adjacent to theside part; a lower surface with a polygonal shape which corresponds tothe upper surface; a side surface disposed between the lower surface andthe upper surface; a first corner cutting edge that is an intersectionof the upper surface and the side surface and is located on the firstcorner part; a second corner cutting edge that is an intersection of theupper surface and the side surface and is located on the second cornerpart; a flat cutting edge that is an intersection of the upper surfaceand the side surface and is disposed on the side part; a major cuttingedge that is an intersection of the upper surface and the side surfaceand is disposed between the second corner cutting edge and the flatcutting edge on the side part; and a connecting edge that is anintersection of the upper surface and the side surface and is disposedbetween the flat cutting edge and the major cutting edge. The flatcutting edge and the major cutting edge have a straight line shape, andthe connecting edge has an outwardly protruding curvilinear shape in atop view. The major cutting edge has an upwardly protruding shape, andcomprises an upwardly protruding curvilinear portion, a first straightline portion extending from the curvilinear portion toward theconnecting edge, and a second straight line portion extending from thecurvilinear portion toward the second corner cutting edge in a sideview. The flat cutting edge, the connecting edge, and the first straightline portion are located on a straight line in the side view.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view schematically showing a state in which a cuttingprocess has been carried out using a conventional cutting insert;

FIG. 2 is a perspective view showing a cutting insert according to anembodiment of the present invention;

FIG. 3 is a side view of the cutting insert shown in FIG. 2;

FIG. 4 is a top view of the cutting insert shown in FIG. 2;

FIG. 5 is a partially enlarged top view of the cutting insert shown inFIG. 4;

FIG. 6 is a sectional view taken along section line A-A in the cuttinginsert shown in FIG. 4;

FIG. 7 is a sectional view taken along section line B-B in the cuttinginsert shown in FIG. 4;

FIG. 8 is a sectional view taken along section line C-C in the cuttinginsert shown in FIG. 4;

FIG. 9 is a perspective view showing a cutting tool according to anembodiment of the present invention;

FIG. 10 is a side view showing the cutting tool shown in FIG. 9;

FIG. 11 is a partially enlarged side view of the cutting tool shown inFIG. 10;

FIG. 12 is a partially further enlarged side view of the cutting toolshown in FIG. 11;

FIG. 13 is a perspective view showing a step of a method ofmanufacturing a machined product according to an embodiment of thepresent invention;

FIG. 14 is a perspective view showing a step of the method ofmanufacturing a machined product according to the embodiment of thepresent invention; and

FIG. 15 is a perspective view showing a step of the method ofmanufacturing a machined product according to the embodiment of thepresent invention.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

<Cutting Insert>

A cutting insert 1 (also referred to simply as “insert 1”) of anembodiment is described below with reference to FIGS. 2 to 8. A longdashed double-short dashed line in FIG. 2 indicates a center axis X ofthe insert 1. FIG. 5 is an enlarged top view, partially cut away, of theinsert 1 shown in FIG. 4. As shown in FIGS. 2 to 8, the insert 1 of thepresent embodiment is an indexable insert, and has a lower surface 2, anupper surface 3, a side surface 4 disposed between the lower surface 2and the upper surface 3, and a cutting edge located along anintersection of the upper surface 3 and the side surface 4. The cuttingedge has a first corner cutting edge 5 a, a second corner cutting edge 5b, a flat cutting edge 6, a major cutting edge 7, and a connecting edge8.

The upper surface 3 has a polygonal shape and has a side part, and afirst corner part and a second corner part that are adjacent to the sidepart. The lower surface 2 has a polygonal shape corresponding to that ofthe upper surface 3. Each of the lower surface 2 and the upper surface 3of the insert 1 in the present embodiment has a quadrangular shape witha plurality of corner parts. The corner parts in the present embodimentare not corners in the strict sense of the word, but have a curved shapein a top view. The term “top view” denotes a state in which the uppersurface 3 of the insert 1 is viewed vertically from above the insert 1.

The first corner cutting edge 5 a is an intersection of the uppersurface 3 and the side surface 4, and is located on the first cornerpart. The second corner cutting edge 5 b is an intersection of the uppersurface 3 and the side surface 4, and is located on the second cornerpart. The flat cutting edge 6, the major cutting edge 7, and theconnecting edge 8 are respectively intersections of the upper surface 3and the side surface 4, and are located on the side part. The majorcutting edge 7 is disposed between the second corner cutting edge 5 band the flat cutting edge 6 on the side part. Therefore, the flatcutting edge 6 is disposed closer to the first corner part than themajor cutting edge 7. The major cutting edge 7 is disposed closer to thesecond corner part than the flat cutting edge 6. The connecting edge 8is a portion of the cutting edge which connects the flat cutting edge 6and the major cutting edge 7. Therefore, the connecting edge 8 isdisposed between the flat cutting edge 6 and the major cutting edge 7 onthe cutting edge.

The flat cutting edge 6 and the major cutting edge 7 have a straightline shape in the top view. The connecting edge 8 located between theflat cutting edge 6 and the major cutting edge 7 has an outwardlyprotruding curvilinear shape. The major cutting edge 7 has a curvilinearportion 71, a first straight line portion 72, and a second straight lineportion 73 in a side view. The term “side view” denotes a state in whichthe side surface 4 of the insert 1 is viewed horizontally from the sideof the insert 1. The curvilinear portion 71 has an upwardly protrudingcurvilinear shape. The first straight line portion 72 extends from thecurvilinear portion 71 toward the connecting edge 8. The second straightline portion 73 extends from the curvilinear portion 71 to the secondcorner cutting edge 5 b. Therefore, the major cutting edge 7 has agenerally upwardly protruding shape. Here, the flat cutting edge 6, theconnecting edge 8, and the first straight line portion 72 are located ona straight line.

The insert 1 has a through hole H that penetrates vertically. Thethrough hole H extends between a central portion of the upper surface 3and a central portion of the lower surface 2. The through hole H is ahole that permits insertion of a screw, and is used for fixing theinsert 1 to the holder by screwing the screw onto the holder. Thethrough hole H has a circular shape in the top view, and has a diameterof, for example, 2-12 mm.

Thus, the through hole H extends between a center of the upper surface 3and a center of the lower surface 2, and hence a center axis X of thethrough hole H extends vertically. A virtual plane S orthogonal to thecenter axis X is set in the following in order to evaluate positions ofcomponents in a vertical direction in the insert 1 of the presentembodiment. When the upper surface 3 and the lower surface 2 have thequadrangular shape as in the insert 1 of the present embodiment, anintersection point of diagonals on the upper surface 3 is the center ofthe upper surface 3, and an intersection point of diagonals on the lowersurface 2 is the center of the lower surface 2.

As a material of the cutting insert 1, there is, for example, cementedcarbide or cermet. Examples of a composition of cemented carbide includeWC-Co manufactured by adding cobalt (Co) powder to tungsten carbide(WC), followed by sintering, WC-TiC-Co manufactured by adding titaniumcarbide (TiC) to WC-Co, and WC-TiC-TaC-Co manufactured by addingtantalum carbide (TaC) to WC-TiC-Co. The cermet is a sintered compositematerial obtained by compositing metal with a ceramic ingredient,specifically, a titanium compound composed mainly of titanium carbide(TiC) or titanium nitride (TiN).

A surface of the insert 1 may be coated with a film by chemical vapordeposition (CVD) method or physical vapor deposition (PVD) method.Examples of a composition of the film include titanium carbide (TiC),titanium nitride (TiN), titanium carbonitride (TiCN), and alumina(Al₂O₃).

The lower surface 2 or the upper surface 3 of the insert 1 has a maximumwidth of 5-20 mm. A height from the lower surface 2 to the upper surface3 is 2-8 mm. The shapes of the upper surface 3 and the lower surface 2are not limited to the above embodiment. For example, the shape of theupper surface 3 in the top view may be a polygonal shape, such astriangle, pentagon, hexagon, or octagon.

The upper surface 3 has a land surface 31, a rake surface 31, and a flatsurface 33 as shown in FIG. 2. The land surface 31 is continuous with acutting edge corresponding to an outer edge of the upper surface 3. Therake surface 32 is disposed in a region closer to the through hole Hthan the land surface 31 as shown in FIGS. 6 and 7. The rake surface 32is an inclined surface, whose height decreases toward the through holeH. The flat surface 33 is disposed in a region closer to the throughhole H than the rake surface 32 as shown in FIGS. 7 and 8. The flatsurface 33 is a flat surface orthogonal to the center axis X along apenetrating direction of the through hole H. The lower surface 2 in thepresent embodiment is a flat surface orthogonal to the center axis X.

The land surface 31 is continuous with the cutting edge and is disposedin a region closer to the through hole H than the cutting edge. The landsurface 31 is a belt-shaped surface region with a small width disposedalong the cutting edge as shown in FIG. 2 or 4. A portion of a region ofthe land surface 31 of the present embodiment, which is close to thesecond corner cutting edge 5 b, is increased in height toward the centerof the upper surface 3 as shown in FIG. 8.

A cutting edge is formed along an intersection of the land surface 31and the side surface 4. The land surface 31 is disposed to enhancestrength of the cutting edge. The rake surface 32 located inside theland surface 31 is an inclined surface, whose height decreases towardthe center of the upper surface 3, as shown in FIG. 6. Therefore, aninternal angle formed by the rake surface 32 and the side surface 4 issmall. However, an internal angle formed by the land surface 31 and theside surface 4 is larger than the internal angle formed by the rakesurface 32 and the side surface 4. Accordingly, the land surface 31contributes to enhancing the strength of the cutting edge. A width ofthe land surface 31, which is indicated by a distance between the outeredge of the upper surface 3 and an outer edge of the rake surface 32, issuitably set according to cutting conditions, specifically, in a rangeof 0.05-0.5 mm.

The rake surface 32 is continuous with the land surface 31, and isdisposed in a region closer to the center axis X than the land surface31. The rake surface 32 is a surface, along which chips cut by the majorcutting edge 7 graze. Consequently, the chips of a workpiece flow alonga surface of the rake surface 32. The rake surface 32 is an inclinedsurface, whose height decreases toward the center of the upper surface 3in a region shown in FIG. 6 or 7, in order to achieve satisfactory chipdisposal.

Although not particularly shown in the drawings, an inclination angle,which is indicated by an angle formed by the lower surface 2 and therake surface 32 in a cross section perpendicular to the rake surface 32,may be set in a range of 5°-30°. The rake surface 32 needs to bedecreased in height toward the center of the upper surface 3. Therefore,the rake surface 32 may be made up of a plurality of regions havingdifferent inclination angles or, alternatively, may have a concavecurvilinear shape.

The rake surface 32 has a smaller height than the flat surface 33between the second corner cutting edge 5 b and the through hole H in aregion shown in FIG. 8. Hence, chips cut by the major cutting edge areless apt to be caught in a region of the rake surface 32 between thesecond corner cutting edge 5 b and the through hole H. That is, it isless liable to induce chip clogging between the insert 1 and theworkpiece, which is caused by the chips being caught in the region. Thismakes it possible to suppress fractures of the insert 1 due to packedchips.

The flat surface 33 is located further inside the upper surface 3 thanthe rake surface 32, and is disposed in a region closer to the throughhole H than the rake surface 32. The through hole H is disposed in aregion closer to the center axis X than the flat surface 33. Owing tothe flat surface 33 thus disposed, the insert 1 is fixable to the holderusing a retainer (not shown) that is called clamper. Specifically, thedamper and the holder holds therebetween the lower surface 2 and theflat surface 33 of the insert 1, and the insert 1 is fixable between theclamper and the holder.

On this occasion, the flat surface 33 is preferably located lower thanthe major cutting edge 7 as shown in FIG. 7. Chips are less apt tocontact with the flat surface 33 because the flat surface 33 is locatedlower than the major cutting edge 7. During the time that the majorcutting edge 7 is in contact with the workpiece, the flat surface 33 islocated further behind in a rotation direction of the holder than themajor cutting edge 7. Therefore, chips are less apt to fly off to theflat surface 33, and hence the chips are less apt to attach to the flatsurface 33. In the absence of the chips on the flat surface 33, themajor cutting edge 7 is smoothly replaceable when replacement of themajor cutting edge 7 to be used is carried out by rotating the insert 1relative to the holder.

The flat surface 33 is located higher than the second corner cuttingedge 5 b in the region shown in FIG. 8. When the cutting process iscarried out using the second corner cutting edge 5 b, the flat surface33 is located higher than the second corner cutting edge 5 b. This makesit possible to control so that chips come into contact with an inclinedsurface 34 connecting the rake surface 32 and the flat surface 33. Theinclined surface 34 is disposed at a position that is higher than therake surface 32 and is lower than the fat surface 33. Consequently, itis possible to suppress the attachment of chips to the flat surface 33.

The side surface 4 is disposed between the lower surface 2 and the uppersurface 3. The side surface 4 functions as a flank surface and isconnected to the cutting edge on the outer edge of the upper surface 3.A step is formed on the side surface 4 as shown in FIG. 2 or 3. An upperportion 4 a of the side surface 4 located higher than the step isconnected to the cutting edge in a side view. The upper portion 4 a ismade into a shape with concave and convex parts conforming with theshape of the cutting edge. A lower portion 4 b of the side surface 4located below the step in the side view has flat portions 41 locatedbelow the side part on the upper surface 3, and a curved portion 42 thatis located below the corner part including the first corner part and thesecond corner part on the upper surface 3, and connects different flatportions 41 together. A length of the flat portions 41 along a directionparallel to the lower surface 2 (a right-and-left direction in FIG. 3)in the side view is set in a range of, for example, 5-20 mm, and alength thereof along a direction orthogonal to the lower surface 2 (anup-and-down direction in FIG. 3) in the side view is set in a range of2-8 mm.

Owing to the flat portions 41 disposed on the side surface 4, aplurality of kinds of cutting inserts are attachable to the holder ifthe flat portions 41 in the lower portion 4 b of the side surface 4 havethe same shape, only by suitably changing the shape of the upper portion4 a of the side surface 4 without changing the shape of the holder.Consequently, the workpiece is machinable into a desired shape only byselecting a necessary cutting insert and then attaching it to the holderaccording to the material or size of the workpiece. That is, it isunnecessary to replace the holder with one corresponding to the cuttinginsert. This simplifies a method of manufacturing a machine product,thus improving producibility of the machined product.

The insert 1 of the present embodiment has a quadrangular shape and hasfour side parts and four corner parts in the top view. The two cornerparts are adjacent to each other as a pair on each of the side parts.Accordingly, the insert 1 of the present embodiment has four flatcutting edges 6, four major cutting edges 7, and four connecting edges8. The corner cutting edges are respectively disposed between the majorcutting edges 7 along the outer edge of the upper surface 3.Accordingly, the four corner cutting edges, including the first cornercutting edge 5 a and the second corner cutting edge 5 b, are disposed.

In the insert 1 of the present embodiment, on the basis of one of thefour side parts, a pair of corner parts adjacent to the side part isreferred to as a first corner part and a second corner part. A cornercutting edge located on the first corner part is referred to as thefirst corner cutting edge 5 a. The corner cutting edge located on thesecond corner part is referred to as the second corner cutting edge 5 b.

Although the four flat cutting edges 6, the four major cutting edges 7,the four connecting edges 8, and the four corner cutting edges aredisposed because the insert 1 of the present embodiment has thequadrangular shape in the top view, the insert 1 is not limited thereto.The number of the flat cutting edges 6, the number of the major cuttingedges 7, the number of the connecting edges 8, and the number of thecorner cutting edges may be three, five, or 6 or more according to thepolygonal shape of the insert 1 in the top view.

In a cutting tool 100 shown in FIG. 9 or 10, using the insert 1 of thepresent embodiment, one of the four major cutting edges is used for acutting process of the workpiece. When the major cutting edge being usedis deteriorated due to the cutting process for a long time, the insert 1may be temporarily removed from the holder 101, and thereafter, theinsert 1 may be reattached to the holder 101 by rotating the insert 1 by90° relative to the center axis X. Thus, any other unused major cuttingedge is usable for the cutting process of the workpiece.

Although the intersection of the upper surface 3 and the side surface 4is not particularly shown in the drawings, the intersection does nothave a strict line shape formed by the intersection of two surfaces. Thedurability of the cutting edge can deteriorate when the intersection ofthe upper surface 3 and the side surface 4 is sharpened at an acuteangle. Therefore, a so-called horning process may be carried out toimpart a slight curved surface shape to a region where the upper surface3 and the side surface 4 intersects with each other.

Each of the flat cutting edges 6 is disposed on the side part of theupper surface 3. Specifically, the flat cutting edge 6 corresponds tothe side part between the first corner cutting edge 5 a and the secondcorner cutting edge 5 b, and is disposed closer to the first cornercutting edge 5 a. As shown in FIG. 5, the flat cutting edge 6 has astraight line shape in the top view. With the insert 1 attached to theholder, the flat cutting edge 6 is approximately parallel to themachining surface of the workpiece and is not inclined even to afinished surface. Therefore, a flat finish is achievable by reducing theconcave and convex parts on the finished surface. The term“approximately parallel” includes those being inclined at an angle of0.5° or less. A length of the flat cutting edge 6 is set in a range of1-3 mm.

The first corner cutting edge 5 a and the second corner cutting edge 5 bhave an outwardly protruding curvilinear shape and are respectively madeup of three regions R1, R2, and R3 having different radii of curvaturein the top view as shown in FIG. 5. The radius of curvature in theregion R1 is set to 5-50 mm. The radius of curvature in the region R2 isset to 0.5-1 mm. The radius of curvature in the region R3 is set to1.4-2.5 mm. A dimensional relationship of the radii of curvature in R1to R3 is established as follows. That is, the radius of curvature in R2is smaller than the radius of curvature in each of R1 and R3, and theradius of curvature in R3 is smaller than the radius of curvature in R1.Namely, the radii of curvature in R1, R2, and R3 are set to satisfy thefollowing relationship: R2<R3<R1.

A portion of the second corner cutting edge 5 b, at which a cutting edgeangle with respect to the workpiece changes significantly, is apt to befractured. The major cutting edges 7 adjacent to each other are gentlyconnectable with a large circular arc by disposing the region of R2 ofthe second corner cutting edge 5 b, which has a small radius ofcurvature, in the center of the second corner cutting edge 5 b. Thismakes it possible to suppress fractures in the second corner cuttingedge 5 b in the insert 1 of the present embodiment.

As shown in FIG. 3, the flat cutting edge 6 has a straight line shapeand is inclined upward from the first corner cutting edge 5 a to theconnecting edge 8 in the side view. The flat cutting edge 6 has asmallest vertical height at a portion thereof being connected to thefirst corner cutting edge 5 a. The corner cutting edges are set to anidentical height, and a portion of the cutting edge which has thesmallest height is located at the corner cutting edge. This ensures thata height position of the rake surface is lower than a height position ofthe flat surface between the corner cutting edges and the through hole.Consequently, it is possible to reduce the possibility that chips arecaught in the region between the corner cutting edges and the throughhole as described above.

The major cutting edge 7 is disposed on the side part of the uppersurface 3. Specifically, the major cutting edge 7 is disposed betweenthe first corner cutting edge 5 a and the second corner cutting edge 5b, and is disposed closer to the second corner cutting edge 5 b. Themajor cutting edge 7 is used as a major edge during cutting of theworkpiece. The major cutting edge 7 has a straight line shape in the topview as shown in FIG. 5.

The major cutting edge 7 has the curvilinear portion 71, the firststraight line portion 72, and the second straight line portion 73 in theside view. The curvilinear portion 71 has an upwardly protrudingcurvilinear shape. The first straight line portion 72 extends from thecurvilinear portion 71 toward the connecting edge 8. The second straightline portion 73 extends from the curvilinear portion 71 toward thesecond corner cutting edge 5 b. Therefore, the major cutting edge 7 hasa top portion 7 b and has a generally upwardly protruding bent shape inthe side view as shown in FIG. 3.

The major cutting edge 7 does not have a generally straight line shapebut an upwardly protruding bent shape in the side view. The entirety ofthe major cutting edge 7 is inclined relative to the workpiece. Thismakes it easier to contact with the workpiece than the case where themajor cutting edge 7 has a straight line shape parallel to the uppersurface 3. It is therefore possible to reduce cutting resistance of themajor cutting edge 7, thus leading to a satisfactory machining of theworkpiece.

The following description is made assuming that all of the major cuttingedges 7 have the straight line shape. Strong chatter vibration occurs inthe insert 1 when the major cutting edges 7 of the insert 1 start tobite into a surface of the workpiece. When all of the major cuttingedges 7 simply have the straight line shape, all of the major cuttingedges 7 may concurrently start to come into contact with the workpiecedepending on an angle at which the major cutting edges 7 start to comeinto contact with the workpiece. On that occasion, a large impact isapplied to the insert 1 and may cause the chatter vibration. Theoccurrence of the chatter vibration makes it difficult to improvecutting conditions, thus failing to improve machining efficiency.

If all of the major cutting edges 7 have the upwardly protrudingcurvilinear shape, the major cutting edges 7 may start to come intopoint-to-point contact with the workpiece. However, when the majorcutting edges 7 start to bite into the workpiece, a full length of themajor cutting edges 7 is longer than the case where all of the majorcutting edges 7 have the straight line shape. Therefore, when all of themajor cutting edges 7 have the upwardly protruding curvilinear shape,the insert 1 is continuously subjected to a strong impact for a longtime.

In the insert 1 of the present embodiment, each of the major cuttingedges 7 has a top portion 7 b and has an upwardly protruding bent shapein the side view. Therefore, all of the major cutting edges 7 do notconcurrently come into contact with the workpiece when the major cuttingedges 7 start to bite into the workpiece. The full length of the majorcutting edges 7 can be decreased than the case where all of the majorcutting edges 7 have the upwardly protruding curvilinear shape. Hence,the insert 1 of the present embodiment is capable of reducing cuttingresistance and relaxing the impact, thereby suppressing an increase incutting resistance when the insert 1 starts to bite into the workpiece.The second straight line portion 73 is preferably longer than the firstline portion 72 in the side view in order to ensure that the curvilinearportion 71 stably bites into the workpiece even when a cutting depth isrelatively small.

In each of the major cutting edges 7, the curvilinear portion 71 islocated closer to the second corner cutting edge 5 b than the firstcorner cutting edge 5 a, and the top portion 7 p that is locateduppermost is disposed closer to the second corner cutting edge 5 b asshown in FIG. 3. Specifically, a distance between the top portion 7 pand the first corner cutting edge 5 a in a direction parallel to avirtual plane S is longer than a distance between the top portion 7 pand the second corner cutting edge 5 b in the direction parallel to thevirtual plane S in the side view of the insert 1.

The top portion 7 p lies, for example, 0.2-1.2 mm above a heightposition of the first corner cutting edge 5 a and a height position ofthe second corner cutting edge 5 b. A length of the major cutting edge 7in the direction parallel to the virtual plane S is 5-25 mm, and the topportion 7 p is shifted 0.5-1.5 mm toward the second corner cutting edge5 b with respect to the center axis X in the side view as shown in FIG.3. The first straight line portion 72 extending from one end of themajor cutting edge 7, which is connected to the connecting edge 8, tothe curvilinear portion 71 has the straight line shape, and the secondstraight line portion 73 extending from the curvilinear portion 71 tothe other end thereof, which is connected to the second corner cuttingedge 5 b, has the straight line shape. An inclination angle of the firststraight line portion 72 with respect to the virtual plane S is 7-11®,and an inclination angle of the second straight line portion 73 withrespect to the virtual plane S is 4-6° in the side view.

The connecting edge 8 is disposed between the flat cutting edge 6 andthe major cutting edge 7 on the side part of the upper surface 3. Theconnecting edge 8 connects the flat cutting edge 6 and the major cuttingedge 7 together. The connecting edge 8 has an outwardly protrudingcurvilinear shape in the top view. The connecting edge 8 having thecurvilinear shape allows the flat cutting edge 6 to be approximatelyparallel to the machining surface of the workpiece, and also allows themajor cutting edge 7 to be inclined relative to the machining surface ofthe workpiece. The connecting edge 8 of the present embodiment has acircular-arc shape. Although a radius of curvature of the connectingedge 8 is not particularly limited, it is set to, for example, from 0.5mm to 4 mm.

The flat cutting edge 6 and the major cutting edge 7 respectively havethe straight line shapes in the top view. Accordingly, virtual straightlines respectively along the flat cutting edge 6 and the major cuttingedge 7 are set. The connecting edge 8 located between the flat cuttingedge 6 and the major cutting edge 7 has the outwardly protrudingcurvilinear shape. Accordingly, the connecting edge 8 is located awayfrom these virtual straight lines. That is, boundaries between a portionlocated on the virtual straight lines and a portion away from thevirtual straight lines correspond to a boundary between the flat cuttingedge 6 and the connecting edge 8, and a boundary between the majorcutting edge 7 and the connecting edge 8. These contribute to clarifyinga range of the connecting edge 8. The radius of curvature of theconnecting edge 8 is evaluable by selecting optional three points on theconnecting edge 8 in the top view, and then setting a virtual circlepassing through these points.

The connecting edge 8 has the straight line shape in the side view. Theflat cutting edge 6, the connecting edge 8, and the first straight lineportion 72 of the major cutting edge 7 are located on a single straightline. A chip thickness generated by the flat cutting edge 6 and a chipthickness generated by the major cutting edge 7 are different from eachother. Therefore, load is liable to concentrate at the connecting edge 8connecting the flat cutting edge 6 and the major cutting edge 7.However, a change between a cutting resistance exerted on the flatcutting edge 6 and a cutting resistance exerted on the major cuttingedge 7 is reducible by the fact that the flat cutting edge 6, theconnecting edge 8, and the first straight line portion 72 are located onthe single straight line. This makes it possible to reduce the loadconcentration at the connecting edge 8.

The insert 1 of the present embodiment is to be attached to the holder101 so that the first corner cutting edge 5 a, the flat cutting edge 6,the connecting edge 8, and the major cutting edge 7 project partiallybeyond a front end surface of the holder 101 toward the workpiece asshown in FIGS. 10 to 12. Here, the insert 1 is attached to the holder101 so that an axial rake has a positive value. When the major cuttingedge 7 starts to come into contact with the workpiece, a contact areabetween the workpiece and the major cutting edge 7 can be minimizedowing to the top portion disposed on the major cutting edge 7. FIG. 12shows part of FIG. 11, and parallel lines PL and PL′ that are parallelto the flat cutting edge 6 are respectively indicated by alternate longand short dashed lines.

The major cutting edge 7 has the straight line shape in the top view ofthe insert 1. However, the axial rake has the positive value. Hence,with the insert 1 attached to the holder 101, an inclination angle ofthe first straight line portion 72 of the major cutting edge 7 is largerthan an inclination angle of the second straight line portion 73 of themajor cutting edge 7 with respect to a plane direction along the flatcutting edge 6 in the side view.

Specifically, with the insert 1 attached to the holder 101, aninclination angle of the first straight line portion 72 with respect tothe parallel line PL along the flat cutting edge 6 and an inclinationangle of the second straight line portion 73 of the major cutting edge 7with respect to the parallel line PL′ have different values as shown inFIG. 12. In the present embodiment, the inclination angle of the firststraight line portion 72, so-called cutting edge angle θ1, is set to14-15°. Setting is made so that the inclination angle of the secondstraight line portion 73, so-called cutting edge angle θ2, is11.5-12.5°. The cutting edge angle is set to less than 20° in the insert1 of the present embodiment. By setting the cutting edge angle to lessthan 20°, even when the workpiece is cut at a high feed rate, a chipthickness is small and thus makes it possible to suppress cuttingresistance. Therefore, the major cutting edge 7 is less apt to generateheat, thereby reducing wear due to the heat of the major cutting edge 7.

The cutting edge angle θ1 of the first straight line portion 72 islarger than the cutting edge angle θ2 of the second straight lineportion 73 in the major cutting edge 7, and it is therefore possible toreduce thrust force exerted on the holder 101. This ensures that whenthe cutting tool 100 cuts the workpiece, a rear edge is less liable tocontact with a finished surface obtained by cutting. Consequently, thefinished surface after being smoothened by the flat cutting edge 6 isless susceptible to damage.

To be specific, when both of the cutting edge angle θ1 of the firststraight line portion 72 and the cutting edge angle θ2 of the secondstraight line portion 73 have a large value, the chip thicknessgenerated by the second straight line portion 73 is large. The secondstraight line portion 73 is subjected to a large impulse of load becausethe second straight line portion 73 is located further away from arotation center axis of the holder 101 than the first straight lineportion 72. Reversely, when both of the cutting edge angle θ1 of thefirst straight line portion 72 and the cutting edge angle θ2 of thesecond straight line portion 73 have a small value, it is necessary toincrease the length of the major cutting edge 7 in order to ensure acutting depth. This increases the thrust force exerted on the holder101.

The cutting edge angle θ1 is larger than the cutting edge angle θ2 inthe present embodiment. Accordingly, the cutting depth is ensured at thefirst straight line portion 72, thereby minimizing the thrust forceexerted on the holder 101. Additionally, the impulse of load exerted onthe second straight line portion 73 can be minimized because the cuttingedge angle θ2 is smaller than the cutting edge angle θ1. Consequently,the damage to the finished surface is suppressible by minimizing thethrust force exerted on the holder 101, while reducing a risk offractures of the cutting edge by reducing the load exerted on the majorcutting edge 7.

Here, the upper portion 4 a of the side surface 4 has a first region 4 a1, a second region 4 a 2, and a third region 4 a 3 as shown in FIG. 3.The first region 4 a 1 is a region of the upper portion 4 a of the sidesurface 4 which is located below the first straight line portion 72. Thesecond region 4 a 2 is a region of the upper portion 4 a of the sidesurface 4 which is located below the second straight line portion 73.The third region 4 a 3 is a region of the upper portion 4 a of the sidesurface 4 which is located below the curvilinear portion 71.

The present invention is not limited to the foregoing embodiment andvarious changes, improvements, or the like may be made therein withoutdeparting from the spirit and scope of the present invention.

<Cutting Tool>

The cutting tool 100 according to an embodiment of the present inventionis described below with reference to FIGS. 9 to 12. FIGS. 9 to 12 show astate in which the insert 1 is attached to an insert pocket 102 (alsohereinafter referred to simply as “pocket 102”) of the holder 101 by ascrew 103. A long dashed double-short dashed line in each of FIGS. 9 and10 indicates a rotation center axis Y1 of the cutting tool 100.

As shown in FIGS. 9 to 11, the cutting tool 100 of the presentembodiment has the rotation center axis Y1, and has the holder 101having a plurality of pockets 102 on an outer peripheral surface closerto a front end thereof, and the inserts 1 to be respectively attached tothe pockets 102.

The holder 101 has an approximately columnar shape whose center ispositioned at the rotation center axis Y1. The holder 101 has theplurality of pockets 102 disposed at irregular intervals on the outerperipheral surface close to the front end of the holder 101.Alternatively, the plurality of pockets 102 may be disposed at regularintervals on the holder 101. The pockets 102 are configured to attachthe insert 1 thereto, and open onto the outer peripheral surface and thefront end surface of the holder 101. The holder 101 is not a strictcolumnar shape as is apparent from the fact that the holder 101 has theplurality of pockets 102.

The insert 1 is to be attached to each of the pockets 102 disposed inthe holder 101. A plurality of the inserts 1 are attached so that partof the cutting edges projects forward beyond the front end of the holder101, namely, projects beyond the front end surface of the holder 101toward the workpiece. Specifically, the inserts 1 are attached to theholder 101 so that part of the first corner cutting edge 5 a, part ofthe flat cutting edge 8, and part of the major cutting edge 7 projectbeyond the front end surface of the holder 101.

Here, the flat cutting edge 6 is fixed to a position most protrudingbeyond the front end surface of the holder 101. Each of the inserts 1 isattached to the pocket 102 so that the flat cutting edge 6 projectsforward beyond the front end surface of the holder 101 as shown in FIG.11. The attachment is carried out so that the flat cutting edge 6 isapproximately parallel to the front end surface of the holder 101.

In the present embodiment, the insert 1 is fixed to the pocket 102 bythe screw 103. That is, the insert 1 is attached to the holder 101 byinserting the screw 103 into the through hole of the insert 1, andinserting a front end of the screw 103 into a screw hole (not shown)formed in the pocket 102 so as to fix the screw 103 into the screw hole.For example, steel or cast iron is usable as a material of the holder101. Among others, high-rigidity steel is preferably used.

<Method of Manufacturing a Machined Product>

A method of manufacturing a machined product according to an embodimentof the present invention is described below with reference to FIGS. 13to 15. FIGS. 13 to 15 show the method of manufacturing a machinedproduct. A long dashed double-short dashed line in each of FIGS. 13 to15 indicates the rotation center axis Y1 of the cutting tool 100. Themachined product is to be manufactured by machining a workpiece. Themethod of manufacturing the machined product according to the presentembodiment has the following steps:

(1) rotating the cutting tool 101 as typified by the foregoingembodiment;

(2) bringing the major cutting edge 7 in the cutting tool 100 beingrotated into contact with the workpiece W; and

(3) separating the cutting tool 100 from the workpiece W.

More specifically, the cutting tool 100 is brought relatively close tothe workpiece W while being rotated around the rotation center axis Y1.Then, the workpiece W is machined by bringing the major cutting edge 7of the cutting tool 100 into contact with the workpiece W as shown inFIGS. 13 and 14. Thereafter, the cutting tool 100 is relatively movedaway from the workpiece W as shown in FIG. 15. The machined product isthus manufactured.

In the present embodiment, the workpiece W is fixed and the cutting tool100 is brought close to the workpiece W. In FIGS. 13 and 14, theworkpiece W is fixed and the cutting tool 100 is rotated around therotation center axis Y1. In FIG. 15, the workpiece W is fixed and thecutting tool 100 is moved away from the workpiece W. Although theworkpiece W is fixed and the cutting tool 100 is moved in each step of amachining process according to the production method of the presentembodiment, the machining process is, of course, not limited to thisembodiment.

For example, in the step (1), the workpiece W may be brought close tothe cutting tool 100. In the step (3), the workpiece W may be moved awayfrom the cutting tool 100. When the machining process is continued, itis necessary to repeat the step of bringing the major cutting edge 7 ofthe insert 1 into contact with different portions of the workpiece W,while the cutting tool 100 is kept rotating. When the major cutting edge7 being used is worn, the major cutting edge 7 which is not used yet maybe used by turning the cutting insert 1 90 degrees with respect to thecenter axis of the through hole. Representative examples of the materialof the workpiece W include carbon steel, alloy steel, stainless steel,cast iron, and nonferrous metals.

DESCRIPTION OF THE REFERENCE NUMERAL

-   1 cutting insert (insert)-   2 lower surface-   3 upper surface-   31 land surface-   32 rake surface-   33 flat surface-   34 inclined surface-   4 side surface-   4 a upper portion-   4 a 1 first region-   4 a 2 second region-   4 a 3 third region-   4 b lower portion-   41 flat portion-   42 curved portion-   5 a first corner cutting edge-   5 b second corner cutting edge-   6 flat cutting edge-   7 major cutting edge-   71 curvilinear portion-   72 first straight line portion-   73 second straight line portion-   7 p top portion-   8 connecting edge-   X center axis-   S virtual plane-   PL, PL′ parallel line-   H through hole-   100 cutting tool-   101 holder-   102 insert pocket (pocket)-   103 screw-   Y1 rotation center axis-   W workpiece

The invention claimed is:
 1. A cutting insert, comprising: an uppersurface with a polygonal shape which has a side part, and a first cornerpart and a second corner part each being adjacent to the side part; alower surface with a polygonal shape which corresponds to the uppersurface; a side surface disposed between the lower surface and the uppersurface; a first corner cutting edge that is an intersection of theupper surface and the side surface and is located on the first cornerpart; a second corner cutting edge that is an intersection of the uppersurface and the side surface and is located on the second corner part; aflat cutting edge that is an intersection of the upper surface and theside surface and is disposed on the side part; a major cutting edge thatis an intersection of the upper surface and the side surface and isdisposed between the second corner cutting edge and the flat cuttingedge on the side part; and a connecting edge that is an intersection ofthe upper surface and the side surface and is disposed between the flatcutting edge and the major cutting edge, wherein the flat cutting edgeand the major cutting edge have a straight line shape, and theconnecting edge has an outwardly protruding curvilinear shape in a topview, wherein the major cutting edge has an upwardly protruding shape,and comprises an upwardly protruding curvilinear portion, a firststraight line portion extending from the curvilinear portion toward theconnecting edge, and a second straight line portion extending from thecurvilinear portion toward the second corner cutting edge in a sideview, and wherein the flat cutting edge, the connecting edge, and thefirst straight line portion are located on a straight line in the sideview.
 2. The cutting insert according to claim 1, wherein thecurvilinear portion is located closer to the second corner cutting edgethan the first corner cutting edge.
 3. The cutting insert according toclaim 1, wherein the second straight line portion is longer than thefirst straight line portion in the side view.
 4. A cutting tool,comprising: a holder having a plurality of insert pockets on a front endpart of the holder; and a cutting insert according to claim 1, which isattached to each of the insert pockets in a state in which the majorcutting edge projects beyond the holder.
 5. The cutting tool accordingto claim 4, wherein, in a state in which the cutting insert is attachedto the insert pocket, an inclination angle of the first straight lineportion of the major cutting edge is larger than an inclination angle ofthe second straight line portion of the major cutting edge with respectto a plane direction along the flat cutting edge in the side view.
 6. Amethod of manufacturing a machined product, comprising: rotating thecutting tool according to claim 4; bringing the major cutting edge ofthe cutting tool being rotated into contact with a workpiece; andseparating the cutting tool from the workpiece.
 7. The cutting insertaccording to claim 2, wherein the second straight line portion is longerthan the first straight line portion in the side view.